This invention pertains to a method of inkjet printing, in particular to a method of inkjet printing designed for high speed, high quality and high resolution. This invention also pertains to an apparatus to perform the inventive method.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on print media, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor.
Inkjet printers offer low cost, high quality printing and have become a popular alternative to other types of printers. However, inkjet printers are presently unable to match the speed of these other printers, especially laser printers.
Most commercial inkjet printers operate with a scanning printhead that moves back and forth over the surface of the print medium printing swathes of the image. The print medium is then advanced step-wise in a direction perpendicular to the scanning direction and the next swath of the image is printed.
Scanning printheads have a number of advantages. For example, a relatively small, inexpensive printhead can be used thus keeping printer costs low. In addition the printhead can be made to scan multiple times over the same area of the substrate to allow slow build up ink for reduced color-to-color bleed, and to allow multiple layers of ink to be applied for increased optical density and chroma. The print medium can also be advanced in increments smaller than the width of the printhead swath and this can be used to hide non-functioning print nozzles by printing over the same area of the print medium with different print nozzles. The primary disadvantage of the scanning printhead is the time that it takes to perform the multiple scans required to cover the whole of the print medium. This time severely limits the throughput of scanning printheads to less than 20 pages per minute.
Full-width arrays have been proposed as one means to address the throughput limitation. In a full-width array, the printhead is at least as wide as the print medium so an image can be printed in a single pass with the substrate moving under the printhead in a direction perpendicular to the array. Historically full-width arrays have not been widely used because of the expense of making a full-width array printhead and the difficulty of getting reliable and uniform jetting from every nozzle of the printhead. However improvements in manufacturing processes and the development of new inkjet printhead technologies have now made full-width array printheads commercially and technically viable.
In a full-width array, the order in which the different colored inks are printed is determined by the printhead design and, unlike a scanning printhead, this order cannot be reversed because there is no return scan. In a reciprocating scanning printhead, any ink can be printed either under or over any other ink.
Ink jet inks are predominantly aqueous based. Colorants that are soluble (dyes) in an aqueous ink vehicle also tend to be resolubilized by water after printing. In other words, images printed with dyes generally lack waterfastness.
One method for making dye-based inks images more waterfast is to treat the image with a “fixing” solution. This fixing solution is usually colorless and can be jetted onto the substrate just like a typical inkjet ink. U.S. Pat. No. 5,723,179 (the disclosure of which is incorporated by reference herein for all purposes as if fully set forth) shows that under-printing of dye-based inks with a fixer can make images water-fast; however, under-printing can decrease the optical density of the images. Also, fixers can cause dull or “muddy” secondary colors.
U.S. Pat. No. 6,621,350 (the disclosure of which is incorporated by reference herein for all purposes as if fully set forth) discloses how aqueous ink jet inks made with water-soluble dyes and vesicle forming surfactants (such as lipids) can be printed on top of an oppositely charged fixer fluid to improve waterfastness with no reduction, or in some cases a slight increase, in chroma.
Printing methods and printhead configurations that can be employed in printers with scanning printheads are not available to or suitable for printers with fixed-array printheads. There is a need for printing methods with array printers that provide good image fastness, especially waterfastness, without negatively affecting other image quality attributes.